Various types of apparatus for removing particles from a dry fluid flow using electrostatic separation techniques are well known in the art. An early example of such an apparatus is shown in U.S. Pat. No. 3,493,109 to Carta et al., the operation and limitations of which are described in detail in commonly assigned U.S. Pat. No. 5,755,333 to Stencel et al., issued May 26, 1998. Generally speaking, the Carta et al. patent relies upon turbulent flow and particle-wall contact in the separation chamber to electrostatically charge the particles. The particles are then drawn from the flow by opposed electrically conductive plates having opposite polarities.
While the apparatus proposed in the Carta et al. reference is somewhat effective for separating particles having a selected charge from a particle mixture, several significant limitations remain. For instance, no effective means is disclosed to ensure that once separated, the selected particles will be directed to the appropriate collection device. To the contrary, the apparatus disclosed in the Carta et al. patent promotes turbulent flow in the separation chamber, which can allow deleterious re-mixing of the particles to occur after separation. As should be appreciated, this reduces efficiency to the point that several cycles or passes through the apparatus may be required to achieve separation. In addition to reducing efficiency, multiple passes significantly increase the particle abrasion to which the wall of the apparatus is subjected thereby reducing the service life of the separator.
In an effort to overcome this shortcoming, commonly assigned U.S. patent application Ser. No. 08/726,255, entitled "Apparatus and Method for Triboelectrostatic Separation," proposes an improved apparatus for separating two species of particles from a particle mixture with greater efficiency and effectiveness by using a curtain gas flow to carry the selected particles drawn from the mixture to a collector for recovery. Similar to the apparatus proposed in the Carta et al. patent, separation is effected through the use of oppositely charged conductor plates connected to a variable voltage source. The charged plates attract oppositely charged particles away from the mixture and towards the sidewalls of the separation chamber. The curtain gas flow (which is initially devoid of particles) is then introduced into the separation chamber to provide the cleaning action necessary to remove or sweep the particles from the plates for recovery.
While this apparatus is effective for separating two particle species from a particle mixture, it should be appreciated that further improvements in separation effectiveness and operational efficiency are still possible. More specifically, there is a need for an electrostatic separation apparatus that: (1) reduces turbulence in the separation chamber(s) to ensure that more selected particles are separated from the particle mixture and collected for recovery; (2) includes separators having one or more relatively compact modular separation stages that each include a pair of separation chamber(s) having one or more elongate electric field zones which are capable of handling relatively high flow velocities to allow for an increase in the amount of the particle mixture processed per unit of cross-sectional area; and/or (3) includes a plurality of separators arranged in parallel to ensure that the particle species are fully separated in a single pass to improve operating efficiency and greatly increase the amount of the particle mixture processed in a given time period.